Zirconium phosphate (ZrP) particles are used as ion exchange materials and are particularly useful as a sorbent material for regenerative dialysis. Zirconium phosphate (ZrP) particles can be synthesized by a sol gel process using zirconium oxychloride (ZOC), also called zirconyl chloride, as a starting material. ZOC is a preferred starting material because it is abundant and commercially available at a low price.
Sol gel precipitation, as the term is used herein, refers generally to a process for forming a ceramic or catalyst in which colloidal particles (called sol) are formed by reacting hydrated metal ions (group III and IV) with a precipitating agent, followed by the polymerization of the colloidal particles to form gel particles. See, for example, Bogdanov S G et al., Structure of zirconium phosphate gels produced by the sol-gel method, J. Phys.: Condens. Matter 9 4031-4039 (1997), incorporated herein by reference. Sol gel precipitation is a particularly advantageous method of obtaining zirconium phosphate from zirconium oxychloride since it is a direct, single-step conversion process that can be carried out at room temperature. Hence, it offers great advantages in efficiency and manufacturing costs in comparison with other processes. Moreover, zirconium phosphate particles obtained by sol gel precipitation generally have a high porosity and a high BET surface area, which enhances their adsorption capacity for ammonia. Further, the use of the sol gel precipitation method allows for control over particle size and morphology of the product, as well as control over impurity levels. These characteristics for zirconium phosphate particles are important with respect to ammonia adsorption and cartridge design for dialysis applications.
Despite all of these advantages, the sol gel precipitation is not easy to accomplish on a manufacturing scale. The difficulties are mainly caused by the nature of the raw material (e.g., ZOC), the rapid rate of the reaction, which is difficult to control, and the lack of appropriate process control methods (flow rate, agitation rate, concentration, etc.). These difficulties can be described as follows.
Sol gel zirconium phosphate, when precipitated directly from zirconium oxychloride solution using phosphoric acid as a precipitating agent, is in the form of soft gel particles having a wide range of particle sizes. One reason why this happens is that zirconium ions in a solution of zirconium oxychloride are highly hydrated monomers, that is, they are surrounded by a large number of coordinated water molecules. During the formation of zirconium phosphate, the soft gel particles tend to agglomerate when the product slurry gets thicker during the reaction process, or when the particles are packed during the filtration and drying process. As a result of this agglomeration, large aggregates are present in the end product after drying so that milling or grinding is required to obtain a free-flowing powder, with the further disadvantage that milling produces a lot of excessively fine particles. Agglomeration also increases the particle size to an extent that is undesirable for column or separation applications.
A wide range of particle sizes in the finished product is a common result of the conventional sol gel precipitation process for the additional reason that the particle size depends on the concentration of the reactants, which gradually decreases as precipitation continues, causing the formation of smaller particles. Thus, it is difficult to control particle size using a single reactant addition technique. Large particles and excessively fine particles are both undesirable for dialysis application because large particles can cause ammonia leakage and smaller adsorption capacity, while fine particles can increase flow resistance and pressure drop in a sorbent cartridge.
Further, the agglomeration of sol gel zirconium phosphate during the precipitation process interferes with agitation and mixing of the reactants as the slurry concentration increases, resulting in the formation of an excessive number of fine particles.
The particle size of sol gel zirconium phosphate can be increased by increasing the amount and concentration of phosphoric acid (for example, by providing a ratio of ZrO2:PO4 of 1:3) but the increase in phosphoric acid also enhances the gelation effect as excess lattice H+ in ZrP combines with H2O molecules.
Accordingly, there is a need for an improved method of synthesizing zirconium phosphate particles that can be carried out on a manufacturing scale.
There is a further need for an improved method of synthesizing zirconium phosphate particles that overcomes one or more of the above-mentioned disadvantages.